Recently, in lens fabrication, mold processes in which a lens is fabricated by transfer of a mold shape are employed more often than polishing. Examples of mold processes include a molding method by hot pressing using glass or a thermoplastic resin as a lens material, a compression molding method or a cast molding method using a thermosetting resin or an ultraviolet curable resin as a lens material, and the like. Mold processes are advantageous in that lenses can be mass-produced at low cost by creating a single mold and aspherical surfaces and free-form surfaces which are difficult to fabricate by polishing can be produced.
As shown in FIG. 16, molds 2 and 3 for molding a lens 1 have lens mold surfaces 2a and 3a which form a lens surface shape such as a spherical surface or an aspherical surface and flat surfaces; 2b and 3b which are formed at the same time as the lens mold surfaces 2a and 3a and which extend from circumferential edges of the lens mold surfaces 2a and 3a. Even when the flat surfaces 2b and 3b are not machined at the same time as the lens mold surfaces 2a and 3a, there are cases where the flat surfaces 2b and 3b are used as machining reference planes for machining the lens mold surfaces 2a and 3a. The lens 1 molded by such molds 2 and 3 has lens surfaces 1a and 1b molded by the lens mold surfaces 2a and 3a of the respective molds 2 and 3 and flange surfaces 1c and 1d which are ring-like flat portions molded by the flat surfaces 2b and 3b and which extend from circumferential edges of the lens surfaces 1a and 1b, As described above, since the flat surfaces 2b and 3b of the molds 2 and 3 function as machining reference planes, inclinations of the lens surfaces 1a and 1b of the molded lens 1 and inclinations of the flat surfaces 2b and 3b of the molds 2 and 3 are equal to each other.
When a lens is molded by a mold process in this manner, if misalignments occur between positions and postures of the two molds mounted on a molding machine, a parallel eccentricity 4 or an inclined eccentricity 5 is created between the molds 2 and 3 as shown in FIG. 17. With the lens 1 shown in FIG. 18 molded using the molds 2 and 3 with such eccentricity, at least one of a parallel eccentricity 6 and an inclined eccentricity 7 of the lens surface 1a occurs with respect to either an external shape of the lens 1 or the lens surface 1b. Depending on the lens, even a parallel eccentricity 6 of around several μm or an inclined eccentricity 7 of around several arc-minutes may prevent desired lens characteristics from being achieved. Therefore, in order to achieve desired lens characteristics, the positions and postures of the mold in the molding machine must be readjusted. To do so, amounts and directions of the parallel eccentricity 6 and the inclined eccentricity 7 of the molded lens must be measured and fed back to the positions and postures of the mold.
From this perspective, for example, methods of measuring amounts and directions of a parallel eccentricity and an inclined eccentricity of a lens and the like have been conventionally proposed. For an inspection of an optical element such as a lens, a mirror, and a prism, a measuring instrument (autocollimator) based on a principle of an autocollimation method which enables precise measurement of an angle of an inspected surface is used.
For example, Patent Document 1 discloses, with respect to an aspherical lens comprising two optical surfaces that form the aspherical lens and two flat portions which are respectively coaxial with the optical surfaces and which are respectively integrally formed with the optical surfaces, a method of detecting an angle of inclination formed between the two flat portions and an angle of eccentricity of the two optical surfaces with respect to a measurement axis in order to compute an eccentricity of the aspherical lens based on detected values thereof.
In addition, Patent Document 2 discloses a method of measuring eccentricity using an autocollimation method even in a case of meniscus lens in which a first surface and a second surface share a same center of curvature.
However, the autocollimation methods used in Patent Document 1 and Patent Document 2 require that a relay lens be appropriately selected according to a radius of curvature of a lens surface of the lens being tested and a spherical wave corresponding to the lens surface be created. Therefore, with autocollimation methods, a dedicated measurement optical system must be prepared for each lens being tested, resulting in inflated facility cost. In addition, a set-up change of measurement optical systems must be performed every time the lens being tested changes, resulting in prolonged measurement preparation periods.
Patent Document 1: Japanese Patent Publication No. 3127003
Patent Document 2: Japanese Patent Application Publication No. H04-106447